Three major forms of PTFE exist: granular, fine powder and micropowder. Granular is produced by suspension polymerization in the absence of surfactant and generally is a spongy, porous irregular particle having a high molecular weight of about 10 million as obtained by rheological creep measurement. Fine powder is coagulated from a dispersion which is polymerized in the presence of an emulsifying agent and has a molecular weight of from about 1 million to about 5 million. Micropowder can be produced as a low molecular weight form of fine powder, or by scission of fine powder products as by gamma or electron beam irradiation. It has a molecular weight of from about 20,000 to about 800,000.
Conventional granular PTFE has an average or mean particle size of from about 20 to about 900 microns and is not melt processable due to its extremely high melt viscosity of at least 108 Pa.s at 380° C. Consequently, PTFE cannot be fabricated by typical melt fabrication processes of extruding and injection molding. Other processes must be utilized such as compression molding and sintering which severely limit the types of articles which can be made.
Fine powder PTFE particles have a mean particle size of from about 20 to about 500 microns and an extremely high melt viscosity of 108pa.s (pascal seconds) at 380° C. and such particles are generally only processable when blended and subjected to shear with a lubricating oil as by extrusion. However, such an extrusion process, known as paste extrusion, requires removal of the lubricant after extrusion.
Prior art patents such as U.S. Pat. Nos. 4,904,735; 5,013,792; and 5,132,368 to Chapman, Jr., et al, of E. I. Du Pont de Nemours and Company of Wilmington, Del., relate to polymer blend compositions containing a major portion of a difficult melt-processible polymer and a minor portion of a fluorocarbon copolymer, or to a fluoropolymer process and wherein the fluoropolymer has a fluorine to carbon ratio of at least 1:2.